Field of the Invention
The present invention relates to an image-capturing optical system, an image-capturing optical apparatus, and a digital device. For example, the present invention relates to: a compact, large-aperture and internal-focus image-capturing optical system that is suitable for an interchangeable lens digital camera that captures an image of a subject with an image-capturing element (for example, a solid image-capturing element such as a Charge Coupled Device (CCD) image sensor, or a Complementary Metal-Oxide Semiconductor (CMOS) image sensor); an image-capturing optical apparatus that outputs the image of the subject captured with the image-capturing optical system and the image-capturing element as an electric signal; and a digital device with an image input function, such as a digital camera, on which the image-capturing optical apparatus is installed.
Description of the Related Art
The market for mirror-less interchangeable lens cameras, which are single-lens reflex cameras from which the jump-up mirror is removed, is expanding because the users accept the compact designs of the cameras. Some of such mirror-less interchangeable lens cameras fail to use phase difference autofocus (AF) that is mainly used in conventional single-lens reflex cameras. Such cameras accordingly use contrast AF in which the camera focuses by scanning the focus group (focusing group) and finding a place for maximum contrast.
In such cameras, the weight of the focus group may be a problem. In the phase difference AF, the travel distance of the focus group required to bring the camera into focus can be calculated using the information from an AF sensor and thus the focus group can move in accordance with the calculated distance. On the other hand, in the contrast AF, the information from the AF sensor includes only the contrast value of the moment. Thus, the camera moves the focus group and reads the variations in contrast during the movement to find a place for maximum contrast and bring the camera into focus. Consequently, the travel distance of the focus group required to bring the camera into focus in the contrast AF is predominantly longer than that in the phase difference AF.
In light of the foregoing, reducing the weight of the focus group is an important point for an image-capturing optical system using contrast AF (for example, see JP 2013-182024 A1 and JP 2013-186458 A1).
The image-capturing optical system disclosed in JP 2013-182024 A1 includes two positive groups and moves the second lens group toward the object side for focusing. However, the second lens group includes four lenses that are a positive lens, a negative lens, a positive lens, and a positive lens. Thus, the weight reduction is not enough to perform the contrast AF. Similarly, the optical system described in JP 2013-186458 A1 includes a first positive lens group and a positive second lens group between which a diaphragm is placed, and moves the second lens group toward the object side for focusing. However, the second lens group includes a lens with a large weight. Thus, the weight reduction of the second lens group is not enough to perform the contrast AF.